This invention concerns a RF filter arrangement useful in a tuner for selecting a desired RF signal while rejecting undesired RF signals, and more particularly with a RF filter arrangement including a trap which rejects the undesired xe2x80x9cimagexe2x80x9d signal associated with the desired RF signal in a tracking relationship as the frequency of the desired signal changes.
A tuner of a television or radio receiver includes a mixer which is intended to combine the RF signal corresponding to the desired channel or station and a local oscillator (LO) signal having a frequency corresponding to the desired channel or station to produce an IF output signal. (For the purpose of this application, the terms xe2x80x9cchannelxe2x80x9d and xe2x80x9cstationxe2x80x9d can be used interchangeably, and, therefore, the term xe2x80x9cchannelxe2x80x9d will be used for convenience.) The mixer produces both a difference frequency component (having a frequency equal to the difference between the frequency of LO signal and the frequency of RF signal) and a sum frequency component (having a frequency equal to the difference between the frequency of LO signal and the frequency of RF signal). The difference frequency component is selected and the sum frequency component is removed by an IF filter to produce an IF signal which is ultimately demodulated. Tuners of television and radio receivers typically employ a tunable RF filter which is coupled between the RF input stage and the mixer and which is tuned in a tracking fashion with the local oscillator. The purpose of the tunable RF signal is to select the RF signal corresponding to the desired channel and to remove unwanted RF signals in order to reduce the presence of unwanted mixer products in the IF signal. In modern television and radio receivers, the local oscillator are electronically tuned in response to a tuning signal, usually a voltage.
One type of unwanted RF signal is the so-called xe2x80x9cimagexe2x80x9d signal. An image signal is an RF signal which is offset from the frequency of the LO signal by the frequency of the IF signal, like the desired RF signal, but in the opposite sense to that of the desired RF signal. Viewed another way, the image signal has a frequency which is offset from the frequency of the desired RF signal by a frequency equal to twice the IF frequency. The frequency of the image signal tracks the frequency of the desired RF signal and the LO signal as the channel is changed. The frequency of the image signal can be above or below the frequency for the LO signal depending on whether the frequency of the LO signal is chosen to be higher than or lower than the frequency of desired RF signal. In a conventional terrestrial broadcast or cable tuner, in which the frequency of the LO signal is chosen to be higher than the frequency of the desired RF signal, the frequency of the image signal is higher than the frequency of the LO signal, as is illustrated in FIG. 3A. In a satellite tuner of the type described in copending U.S. application Ser. No. 08/467,097, entitled xe2x80x9cTuner for a Digital Satellite Receiverxe2x80x9d, filed on Jun. 6, 1995 for M. A. Pugel and K. J. Richter, in which the frequency of the LO signal is chosen to be lower than the frequency of the desired RF signal, the frequency of the image signal is lower than the frequency of the LO signal, as is illustrated in FIG. 3B.
The tunable RF filters employed in tuners are typically bandpass filters. Image signals may be removed or at least reduced in amplitude to the point that they no longer produce objectionable IF responses by employing bandpass filters with so-called xe2x80x9chigh Qxe2x80x9d factors. The xe2x80x9cQxe2x80x9d or xe2x80x9cqualityxe2x80x9d factor of a filter refers to the sharpness of the filter or its ability to select only the desired signal or band of signals and to reject unwanted signals very near the desired signal or band of signals. Viewed another way, the xe2x80x9cQxe2x80x9d of a filter refers to the slope of the filter response between the xe2x80x9cpassxe2x80x9d region and the xe2x80x9crejectionxe2x80x9d region.
A tunable bandpass filter may simply comprise an inductor and a variable capacitor, such as a voltage variable capacitance diode (xe2x80x9cvaractorxe2x80x9d diode). In a typical application, the inductor is connected in parallel with the varactor in what is known as a parallel tuned circuit, and the parallel tuned circuit is connected in-shunt with the path of the signal to be filter. However, such a simple circuit may not be able to adequately reject unwanted signals such as an image signal. A higher Q bandpass filter can be formed by two tuned circuits coupled in cascade. A bandpass filter of the latter type which is commonly employed in tuners comprises two tuned circuits coupled in cascade by virtue of a magnetic coupling between the respective inductors and is known as a doubly-tuned circuit.
In addition to the rejection capabilities of a filter, other considerations are important. It is desirable that the effects of the impedance presented to the filter by a preceding or succeeding processing stage or both be considered. Complexity and its consequential cost are also usually important factors.
The present invention is based in part on the recognition that a relatively simple filter arrangement which performs the functions of (1) selecting a desired signal, (2) rejecting an unwanted signal having a frequency which tracks that of the desired signal, and (3) impedance transformation can be achieved if a first tunable filter for selecting the desired signal is coupled in cascade with a second tunable filter for rejecting the undesired signal, when at least one of the two tunable filters also serves as an impedance transformation device. Such a filter arrangement is particularly well suited for rejecting the unwanted image signal in a tuning system in which the frequency of the LO signal is lower than the frequency of the desired RF signal so that the frequency of the image signal is lower than that of the desired RF signal. These and other aspects of the invention will be described below with respect to the accompanying Drawing.